Rotating anode x-ray tubes are well-known and have been used in medical diagnostic applications for several decades. Originally, rotating anode tubes consisted of an internal tube structure housed in a glass vacuum envelope. More recently, a new category of rotating anode x-ray tube has become available wherein the major portion of the vacuum envelope of the tube is made of metal. So called "metal center" x-ray tubes have the primary advantage of being able to withstand higher power levels, such as those used in modern CT scanning applications.
In a metal center tube, the metal portion surrounds the target portion of the rotating anode and the active, electron-beam producing portion of the cathode structure. Primarily for safety reasons, the metal center section is held at ground potential, whereas both the cathode and anode are held at very high voltages during operation. For example, in a typical rotating anode x-ray tube, whether all glass or metal center, the anode may be held at +75,000 volts and the cathode at -75,000 volts to create a potential difference of 150,000 volts between the electrodes of the tube. (This large potential difference is necessary to impart enough energy to the electron beam that when the electrons strike the anode they have sufficient energy to produce useable x-rays.) In view of these voltage relationships, it is necessary to insulate the grounded metal center section from both the anode and the cathode of the tube.
The insulation of the metal center section is typically accomplished by using glass support cylinders, one for the anode end of the tube and one for the cathode end; although it is also known to use ceramic as an insulator instead of glass. However, the use of ceramic insulating end pieces has been limited to the cathode end of the tube in commercial applications. The insulating end pieces are hermetically sealed to each end of the metal center structure to provide a vacuum tight envelope having electrical isolation between the cathode, metal center structure and the anode.
A major disadvantage of metal center x-ray tubes, not present in glass envelope tubes, has been an electrical weakness in the anode glass region. A frequent cause of failure of metal center tubes is due to electrical discharges where the anode glass flares away from the rotor, (i.e., in the area of curved glass section 83 in FIG. 1). Despite the development of improved parts processing, vacuum firing, tube exhaust and tube aging techniques to avoid this problem, the inherent electrical weakness in this region continues to be a major cause of tube failure, and is becoming a limiting factor in developing even higher power tubes.
Accordingly, an object of the present invention is to provide an improved metal center tube capable of operating at higher power levels.
Another object of the present invention is to reduce the electrical weakness in the anode region of the envelope of metal center x-ray tubes.